The transmission capacity of fiber-optic communication systems has increased significantly with the Wavelength Division Multiplexing (WDM) technique. In a WDM system, multiple channels, where each channel is differentiated by the use of a different wavelength of light, carry modulated optical signals in a single optical fiber. Optical multiplexers are used to combine all the optical channels into the fiber for transmission, and optical demultiplexers are used to separate the optical channels for detection. An additional requirement for optical transmission is that the power level of all the WDM channels be roughly equal, to prevent the corruption of the modulated optical signals on the weaker channels.
In an optical network, network traffic is routed from a plurality of sources to a plurality of destinations via one or more intermediate nodes, each of which may be connected to one or more neighboring nodes. Accordingly, each intermediate node requires some switching or cross connection capability to select an appropriate neighboring node in order to route the traffic towards the desired destination. An intermediate node typically includes opto-electronic conversion and performs wavelength switching or cross connection electronically by (a) terminating one or more wavelength channels at a receiver endpoint, (b) switching wavelength channels through the node, and (c) originating one or more wavelength channels at a transmitter point. Alternatively, the intermediate nodes can operate transparently, routing the individual optical wavelength channels without optoelectronic conversion. Regardless, intermediate nodes are required to switch incoming wavelength channels from an input port to a desired output port, so that a channel originating at an upstream node can pass through the intermediate node en route toward its downstream node destination. When the switching functionality in the intermediate node is performed electronically, the optical transmitters at the output ports can be set to satisfy the power equalization criteria. However, when the switching functionality is operated transparently, the switch has to provide not only the cross-connect feature from input ports to output ports, but must also provide the mechanism to equalize the optical power level at the output ports.